Power semiconductor devices such as power MOSFETs are well known. Power MOSFETs are prevalently used in power control and conversion applications. One well known power application is DC-DC conversion.
DC-DC converters may be used in battery operated devices such as portable computers, portable telephones, and personal digital assistants to regulate the amount of power supplied from the battery to the device. The life of the battery in a portable device depends on the efficiency of its power circuitry. The ever-increasing demands for greater power supply and longer lasting battery power have, therefore, made efficiency in DC-DC converters an important factor for designers.
The efficiency of a DC-DC converter can be improved if certain characteristics of the semiconductor switching devices of the converter are improved. For example, when power MOSFETs are used in a DC-DC converter lowering of the on-resistance and increasing the current capability of the MOSFETs will contribute significantly to the efficiency.
One way to improve the key characteristics of a power MOSFET, for example, the ON resistance of a MOSFET, is to increase the density of the cells of its active area. The increase in the cell density in a power MOSFET, however, may be restricted by the condition of the material used to form the device and the inherent limitations of the process used.
In a typical process for fabricating a trench-type power semiconductor device that includes gate trenches as well as source contact trenches (trenches which allow the source electrode to make electrical contact with the base region) the gate trenches are formed first prior to forming the contact trenches. This process introduces a variability which limits the cell density that can be achieved, thus limiting the current carrying capability that may be attained by increasing the cell density.
It is thus desirable to overcome the limitations of the prior art in order to obtain a device with a higher density of active cells.